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Mechanisms controlling DOC transport; Surface flow vs pipes vs groundwater

water@leeds. Mechanisms controlling DOC transport; Surface flow vs pipes vs groundwater. Pippa Chapman, School of Geography University of Leeds. water@leeds. Sources of DOC in aquatic systems

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Mechanisms controlling DOC transport; Surface flow vs pipes vs groundwater

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  1. water@leeds Mechanisms controlling DOCtransport; Surface flow vs pipes vs groundwater Pippa Chapman, School of Geography University of Leeds

  2. water@leeds • Sources of DOC in aquatic systems • Allochthonous sources – are derived outside of the stream from soil and vegetation within the catchment (terrestrial organic matter) • Autochthonous sources – are derived from in-stream biological production (e.g. algae and macrophytes)

  3. water@leeds • DOC is produced during the decomposition of organic matter in soil • Peat soils contain most organic matter • Major source of DOC • Production of DOC is a biologicalprocess: • Increases with temperature and aeration • So more produced in summer From Hope et al., 1994

  4. water@leeds Transport of DOC in peatlands Overland flow - typically saturation-excess driven. Micropore throughflow - typically close to the surface (top 5 cm). Macropore/tunnel/pipe ‘bypassing’ flow

  5. water@leeds Role of throughflow There is a strong relationship between soil water DOC at 10 cm depth and stream water DOC (r2 = 0.47, P<0.001) at Cottage Hill Sike, Moor House, N Pennines (Clark et al., 2008). This is consistent with hydrological studies at this site that have shown most runoff originates from the top 5 cm (Holden and Burt, 2003). From Clark et al., 2008 Note: despite large fluctuations in discharge little variation in DOC

  6. water@leeds Role of overland flow: based on Cottage Hill Sike from Clark et al., 2007 • DOC concentrations decreased during autumn storms • No relationship observed between DOC and flow • Flux calculation based on weekly ECN sampling was 16% greater than flux based on 4 hourly sampling. • Excluding storm events resulted in over-estimation of DOC flux • 50% of DOC export is associated with the highest 10% of discharge values. Hinton et al (1997) reported similar findings for a catchment in central Ontario, Canada.

  7. water@leeds Soil map of Glendye, NE Scotland (from Dawson) From Hinton et al., 1997. Biogeochem.

  8. Pipeflow runoff pathway coupling Saturation-excess overland flow and near-surface through flow Rapid near-surface macropore and matrix infiltration Blanket peat Very limited deep matrix seepage ? Pipe network Mineral substrate Localised ephemeral interface flow

  9. water@leeds Role of pipeflow

  10. Autosamplers – much more detail water@leeds P5 13 March 2008

  11. Importance of pipeflow in Cottage Hill Sike water@leeds Initial calculations suggest that: • 20-30 % of stream flow originates from pipes • 53 % of the DOC originates from the pipes (typically ranges between 5-75 % depending on conditions) • Pipe probably tap deep/old carbon as well as new carbon (to be further tested with isotopic analysis) • Very variable carbon response between pipes and between storms • Chemistry of at least one pipe suggests that ground water feeds pipe at low flow

  12. water@leeds • Summary • DOC transported from peat to stream by a combination of overland flow, through flow and pipe flow • Response of stream water DOC to storm events dependent on number of source waters and location within the catchment with respect to soil distribution • Transport of DOC from peat and organo-mineral soils is not the same due to differences in hydrology and soil profile properties • Some streams fed by groundwater during low flow – low DOC, pH 6-7. • Largest loss of DOC (~50%) from peat and organo-mineral soils occurs in only 10% of time during high flow events

  13. water@leeds • Acknowledgments • Jo Clark, Joe Holden, Richard Smart, Andy Baird, Mike Billett, Kerry Dinsmore • NERC- funded Jo Clark’s PhD (2000-2004) • NERC - funded Pipe project (2007-2010) • ECN – use of data • Natural England – use of Moor House NNR

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